A probe that is to be used for cryosurgery must be designed with an optimally small shape and size to achieve selective cooling of biological tissues. The cryosurgical system must also be designed to provide reliable cooling of the part of the cryoprobe (i.e. the cryotip) that will be in direct thermal contact with the target biological tissue to be treated.
For many cryogenic treatment applications, temperatures below −90° C. are desirable, and some known cryosurgical systems use liquid refrigerants such as nitrogen, argon, nitrous oxide, carbon dioxide, various hydro/fluorocarbons, and others. Liquid nitrogen has a very desirable low temperature of approximately −200° C., but when it is introduced into the freezing zone of the cryoprobe, where it is in thermal contact with surrounding warm biological tissues, its temperature increases above the boiling temperature (−196° C.). Thus, it evaporates and expands several hundred-fold in volume at atmospheric pressure, and rapidly absorbs heat from the probe tip. This enormous increase in volume results in a “vapor lock” effect when the mini-needle of the cryoprobe gets “clogged” by the gaseous nitrogen. Additionally, in these systems the gaseous nitrogen is typically rejected directly to the atmosphere. This produces a cloud of condensate upon exposure to the atmospheric moisture in the operating room and requires frequent refilling or replacement of the liquid nitrogen storage tank.
Several liquid nitrogen systems have been proposed. For example, improved cryosurgical systems and methods for supplying liquid nitrogen to a probe tip are disclosed in U.S. Pat. No. 5,520,682, and U.S. Pat. No. 7,192,426, both of which issued to Baust et al. Further, a system for the direct and/or indirect delivery of liquid nitrogen to a probe tip is disclosed in U.S. Pat. No. 5,334,181 which issued to Rubinsky et al. For these and other similar type systems, cryosurgical practice shows that current cooling systems and methods that are based on the use of liquid nitrogen as a means to cool a miniature probe tip are not practicably feasible. In large part, this is due to the rapid transition of the liquid nitrogen into the gaseous state followed by an inevitable “vapor lock.”
Nitrous oxide and carbon dioxide systems typically achieve cooling when pressurized gases are expanded through a Joule-Thomson expansion element such as a small orifice, throttle, or other type of flow construction that is disposed at the end tip of the cryoprobe. For example, a typical nitrous oxide system pressurizes the gas to about 5 to 5.5 MPa to reach a temperature of no lower than about −85 to −65° C. at a pressure of about 0.1 MPa. For carbon dioxide, the temperature of about −76° C. at the same pressure of 0.1 MPa is achieved with an initial pressure of about 5.5 MPa. Nitrous oxide and carbon dioxide cooling systems, however, are not able to achieve the temperature and cooling power provided by liquid nitrogen systems. On the other hand, nitrous oxide and carbon dioxide cooling systems have some advantages because the inlet of high pressurized gas at a room temperature, when it reaches the Joule-Thomson throttling component or other expansion device at the probe tip, excludes the need for thermal insulation of the system. However, because of an insufficiently low operating temperature combined with relatively high initial pressure, cryosurgical applications are strictly limited. Additionally, the Joule-Thomson system typically uses a heat exchanger to cool the incoming high pressure gas with the outgoing expanded gas in order to achieve the necessary drop in temperature by expanding compressed gas. Stated differently, these heat exchanger systems are not compatible with the desired miniature size of probe tips that must be less than at least 3 mm in diameter.
Several mixed gas refrigeration systems (e.g. Joule-Thompson systems) have been proposed for performing cryosurgical procedures. In particular, U.S. Pat. No. 5,787,715, U.S. Pat. No. 5,956,958, and U.S. Pat. No. 6,530,234, all of which issued to Dobak, III et al., disclose cryogenic procedures using devices having mixed gas refrigeration systems. Other systems wherein a refrigerant transitions from a liquid to a gas (e.g. a Joule-Thomson effect) include systems disclosed in U.S. Pat. No. 6,074,572 which issued to Li et al. and U.S. Pat. No. 6,981,382 which issued to Lentz et al.
In review, systems using liquid nitrogen as a means to cool a miniature probe tip are subject to “vapor lock.” On the other hand, systems that use highly pressurized gas mixtures in order to achieve the Joule-Thomson effect cannot provide operating temperatures lower than about −90° C. Thus, they are not desirable for many cryosurgical procedures.
In light of the above, an object of the present invention is to provide a closed-loop system for performing a cryosurgical procedure with a cryoprobe that maintains a liquid refrigerant in its liquid state as it transits through the system. More specifically, it is an object of the present invention to provide a system and method for performing a cryoablation treatment that employs non-evaporative liquid refrigerants at a low pressure (e.g. 0.3 MPa), and at a low temperature (e.g. less than −100° C.). It is another object of the present invention to provide a cryoablation system that can be customized to use any one of several different liquid refrigerants. Still another object of the present invention is to provide a cryoablation system that incorporates a means for removing frozen biological tissue that may adhere to the cryoprobe during a cryosurgical treatment. It is also another object of the present invention to provide a cryoablation system that is easy to use, is relatively simple to manufacture and is comparatively cost effective.